The present invention relates to a black chromate solution used for forming a highly anticorrosive black chromate film on a Zn plate, Zn-plating, Zn alloy plating such as Zn-Ni alloy plating, and a method of forming a chromate film thereon, especially on a Zn-Ni alloy.
The treatment of a zinc plating with a chromate is still widely employed for the purposes of decoration and anticorrosion in the fields of automobiles, domestic electric appliances, etc.
However, the plating thus formed has defects that the corrosion resistance thereof is yet insufficient under severe conditions when it is kept in an overheated engine room or on an icy road, when it is exposed to a chemical used for removing snow when it snows, or when it is damaged by salty sea breezes. Under these conditions, various Zn alloy platings capable of forming a plating film having a corrosion resistance far superior to that of a zinc plating were investigated and developed. Among them, a high corrosion resistance of a Zn-Ni alloy attracts attention mainly in the automobile field. As a monotony tint and a high grade are inclined to be liked by the users recently, a substance for blacking the Zn-Ni alloy plating is demanded. In addition, there is investigated a black chromate which does not impair the properties of the Zn-Ni alloy plating and further a chemically stable black chromate solution useful when the Ni deposition rate is in the practical range of 3 to 12% by weight. The best technique comprises adding an aliphatic carboxylic acid to an ordinary black chromate solution for treating Zn to form a black chromate film on the Zn-Ni alloy plating.
However, the process wherein formic acid, citric acid or tartaric acid is used as the aliphatic carboxylic acid has a problem that the treating solution is impractical, since the solution is chemically unstable and the ingredients in the solution tend to be decomposed. The process wherein succinic acid is used has a problem that the appearance thereof is yet insufficient in the blackness and gloss, though the treating solution is chemically stable.